In hybrid motor vehicles, the drive train is generally made up of an internal combustion engine and at least one electric motor. In a parallel hybrid drive train, the internal combustion engine can be coupled to the electric motor via a clutch that can be influenced by a control device. In addition to the operating modes hybrid driving, boost operation, and recuperation mode, an operating mode also results from this that enables purely electrical driving. A starting of the internal combustion engine can for example be required during electrical driving mode if the driver requires more drive power than the electric motor is able to produce, or if the energy content of an electrical energy storage device for the electric motor decreases too strongly. This starting usually takes place through a tow-starting of the internal combustion engine by the electric motor. The internal combustion engine and the electric motor represent two sources of torque that, when the clutch is closed, together produce an overall drive torque referred to as hybrid driving mode. It is not always possible to clearly determine whether, after tow-starting by the electric motor, the internal combustion engine has started without error, i.e. whether combustions are taking place. In particular given low torque requirements on the internal combustion engine, this cannot be determined reliably from the signals that are typically available, e.g. from control devices. A measurement of the torque produced by the internal combustion engine, or of the overall drive torque produced together by the internal combustion engine and the electric motor, usually does not take place for reasons of cost.